The present invention relates to electrical transformers and particularly to a transformer core comprised of grain oriented silicon steel-amorphous steel composite.
Traditionally, electrical transformer cores have been formed of highly grain oriented silicon steel laminations. Over the years, significant improvements have been made in such electrical steel to permit reductions in transformer core size, manufacturing cost and the losses introduced into an electrical distribution system by the transformer core. As the cost of electrical energy continues to rise, reductions in core loss have become an increasingly important design consideration in all sizes of electrical transformers. For this reason, amorphous ferromagnetic materials are being actively considered for use in transformer cores to achieve a significant decrease in core operating losses.
Amorphous metals are principally characterized by a virtual absence of a periodic repeating structure on the atomic level, i.e., the crystal lattice, which is a hallmark of their crystalline metallic counterparts. The non-crystalline amorphous structure is produced by rapidly cooling a molten alloy of appropriate composition such as those described in Chen et al., in U.S. Pat. No. 3,856,513, herein incorporated by reference. Due to the rapid cooling rates, the alloy does not form in the crystalline state, but assumes a metastable, non-crystalline structure representative of the liquid phase from which it was formed. Due to the absence of crystalline atomic structure, amorphous alloys are frequently referred to as "glassy alloys".
Due to the nature of the manufacturing process, an amorphous ferromagnetic strip suitable for application in a laminated transformer core is extremely thin, normally 1-2 mils versus 7-12 mils for grain oriented silicon steel. Moreover, such amorphous steel strips are quite brittle and thus easily fractured. These characteristics render the processing of the amorphous strips into suitable core laminations and the subsequent handling thereof to build a transformer core a most difficult and rather costly procedure. That is, special cutting techniques are required to cut the amorphous steel strips to the desired core lamination sizes. Moreover, for stacked cores, such as utilized in power transformers, conventional lamination stacking, end lamination insertion, and clamping methods utilized in silicon steel laminated cores are not completely satisfactory for amorphous metal laminations because of the basic thinness, brittleness and strain sensitivity of this material. Another and perhaps most significant limitation of amorphous ferromagnetic steel is that it has an approximately 25% lower saturation density than grain oriented silicon steel. Consequently, an amorphous metal core would have to be physically larger than a silicon steel core in order to carry the same level of flux. This factor places a significant economic penalty in order to achieve reduced core losses particularly at higher KVA ratings, since amorphous steel constitutes a higher material cost than silicon steel. Yet another factor requiring a larger amorphous steel core relative to a comparably rated silicon steel core is that the former possesses an inherently lower packing factor, i.e., the ratio of ferromagnetic material cross sectional area in a core member to the overall cross sectional area of the core member.
The foregoing considerations have lead to the general consensus in the power transformer field that the numerous drawbacks of amorphous ferromagnetic steel cores economically outweigh the advantage of reduced core loss achieved therewith. There is however considerable activity with regard to the use of amorphous steel in physically smaller transformers typically applied to the distribution of electrical power as contrasted to the transmission of electrical power which is the realm of the power transformer. For example, U.S. Pat. Nos. 4,364,020 to Lin et al. and 4,520,335 to Rauch et al. disclose wound distribution transformer cores having a combination of amorphous steel and silicon steel laminations distributed throughout the yokes and legs which are joined together to provide one or more magnetic loop circuits consisting exclusively of amorphous steel and one or more magnetic loop circuits consisting exclusively of silicon steel. The same arrangement of parallel amorphous steel and silicon steel flux circuits is disclosed in a stacked core by Lin, U.S. Pat. No. 4,506,248.
It is accordingly an object of the present invention to provide a transformer core having improved core loss characteristics.
A further object of the present invention is to provide a transformer core of the above character which is constructed to utilize amorphous steel laminations in a commercially viable manner.
Another object of the present invention is to provide a transformer core of the above character which is formed of a combination of amorphous steel and silicon steel laminations to achieve a composite core having low loss characteristics.
An additional object of the present invention is to provide a amorphous steel-silicon steel composite laminated transformer core which is efficient in design, improved in manufacturability, and reliable over a long service life.
Other objects of the invention will in part be obvious and in part appear hereinafter.